Water-cooled nuclear reactors and in particular pressurized-water nuclear reactors comprise assemblies consisting of a bundle of fuel rods of considerable length, arranged parallel to each other and held inside a framework formed by guide tubes, struts and two end blocks. The guide tubes are arranged in the longitudinal direction of the assembly and are connected to transverse struts regularly spaced along the length of the assembly.
The guide tubes are also connected at each of their ends to one or other of the two end blocks forming parts for rigidifying and closing the assembly.
The fuel rods of the assembly form a bundle in which the rods are parallel to each other and arranged, in the transverse sections of the assembly, in a regular latticework determined by the struts. Certain positions of the latticework are occupied by guide tubes which are generally rigidly connected to the struts.
The guide tubes have a length greater than the length of the fuel rods and are positioned inside the bundle, so as to comprise a part projecting relative to the bundle of fuel rods at each of their ends. The end blocks are fixed onto these projecting parts of the guide tubes so as to ensure closure of the assembly at each of its ends.
The fuel rods consist of sintered pellets of nuclear fuel material stacked inside a metal sheath isolating the pellets from the fluid surrounding the fuel assembly. In the event of rupture of a sheath of a fuel assembly rod, this rod must be replaced very rapidly in order to avoid leakages of radioactive product into the cooling fluid of the reactor. In order to gain access to the fuel rods and carry out replacement thereof, one of the end blocks of the assembly must be demounted, which assumes the elimination of the connections between the corresponding ends of the guide tubes and the end block.
The end blocks comprise through-holes reproducing the latticework of the guide tubes, in each of which a guide tube is engaged and fixed.
To permit replacement of defective rods in the fuel assemblies new fuel assemblies have been designed and developed, comprising guide tubes, the connection of which with at least one of the end blocks is demountable.
In order to carry out replacement of the defective fuel rods, the assembly is placed under water in the vertical position, inside a well such as a storage well; the assembly rests on the bottom of the well via one of its end blocks or bottom end block. The other end block or top end block is accessible at a certain water depth from the top of the well.
In a known type of demountable fuel assembly, the parts of the guide tubes engaged in the top end block of the assembly comprise a radially expandable part which may be, for example, mounted on the end of the guide tube. This expandable part may consist of a slitted bush having a part radially projecting outwards which is intended to be accommodated inside a cavity of corresponding shape machined inside the end block, in the through-hole of the guide tube. A locking sleeve introduced inside the guide tube effects radial expansion of the slitted bush and securing of the guide tube, the radially projecting part of which is accommodated inside the cavity machined in the end block.
Only a certain length of the guide tube is engaged in the hole passing through the adapter plate of the end block, the remaining part of the hole, above the guide tube, emerging on the upper surface of the adapter plate of the end block.
A demountable connection for the guide tube of a fuel assembly of the type described above is known, comprising a locking sleeve having a bush ensuring expansion of the guide tube extended axially by a fixing ferrule which is accommodated, when the locking sleeve is arranged in position inside the guide tube, in the part of the hole situated above the guide tube and emerging on the upper surface of the adapter plate. Radial cavities are provided in this part of the hole of the adapter plate and the fixing ferrule is deformed, after arranging the locking sleeve in position inside the guide tube, such that the deformed parts of this fixing ferrule fit inside the cavities so as to achieve axial and rotational locking of the locking sleeve.
Effective fixing of the guide tube is thus obtained by means of operations which may be carried out, without difficulty, from the top of the assembly.
However, demounting of the guide tube requires that, initially, the locking sleeve which is held inside the end block by the fixing ferrule be extracted. This operation may be performed by a tool which is introduced into the sleeve and which comprises parts, movable in radial directions, which are positioned under the bottom end of the sleeve. A pulling force is exerted on the tool so as to allow the fixing ferrule to be unlocked and the sleeve of the guide tube to be extracted.
This operation of extracting the locking sleeve before demounting the top end block of the assembly requires the use of a complex tool and the application of pulling forces which are all the greater the more effectively the sleeve is fixed by means of the ferrule.